Multiple-rotor electric machines are known. Such machines are discussed, for example, in U.S. Patent Application Publication No. 2005/0140230 of Johnson et al.; in U.S. Pat. No. 5,534,737 to Nishimura; in German Patent Publication DE 3234673; and in Japanese Patent Number JP 09046969.
However, prior art multiple-rotor machines such as those discussed in the cited references are heavier and less powerful than necessary or desirable, and may provide relatively unsteady power output.
There is a need for lighter and more powerful electric machines, which are capable of outputting steady and stable power.